1. Field of the Invention
This invention relates to a process for producing a solution of an epoxy resin as a lacquer, and more specifically, to a process by which a lacquer containing an epoxy resin component having a uniform molecular weight and being in the form of a solution directly usuable as a lacquer is produced at a high production speed and in a high yield.
2. Description of the Prior Art
Epoxy resins for paints, particularly lacquers for coating metals such as metallic containers, are desired to have a relatively high molecular weight of about 2,000 to about 10,000 in order to increase the processability or the curing speed of coated films.
Conventional methods of producing such epoxy resins for lacquers fall roughly within two groups: a one-step process and a two-step process.
The one-step process involves directly reacting bisphenol A [2,2-bis(4-hydroxyphenyl)propane] and an epihalohydrin in the presence of an acid binder. The resulting epoxy resin, however, should be subjected to a troublesome purifying operation such as washing, and its molecular weight is generally low.
The two-step process comprises first preparing a liquid epoxy resin having about one bisphenol A skeleton on an average in the molecular chain from bisphenol A and an epihalohydrin, and then reacting the liquid epoxy resin with bisphenol A in the presence of a catalyst under molten conditions to produce an epoxy resin having a high molecular weight. Since this melt polyaddition process gives the final epoxy resin in the form of a solid mass, its solution for a lacquer must be prepared by subjecting it to a troublesome dissolving operation which requires high temperatures of about 80.degree. to 100.degree. C. and long periods of several hours. Furthermore, the resulting epoxy resin is very highly viscous in the molten state and remains adhering to the reactor wall. Only about 80% of it can be recovered (that portion adhering to the reactor wall is removed by washing with a solvent), and its yield is not satisfactory. Moreover, in the second melt polyaddition step, the reaction inevitably proceeds heterogeneously, and it is impossible to avoid inclusion in the final epoxy resin of a certain proportion of an epoxy resin component having a relatively low molecular weight of about several hundred or a gelled component whose molecular chain structure is crosslinked. This is undesirable in view of the various properties of the resin used as a lacquer, such as extraction resistance, processability and corrosion resistance.
To remove these defects, it may be possible to react a liquid epoxy resin and bisphenol A in an organic solvent thereby directly producing an epoxy resin as a solution which is easy to handle. However, when the solids concentration of the solution is adjusted to a value which makes the solution easy to handle in respect of its viscosity, the polyaddition reaction between the liquid epoxy resin and bisphenol A takes place at a markedly slow rate, and the production speed is not satisfactory. Furthermore, the molecular weight of the final epoxy resin tends to become relatively broad.